A flowmeter of particular background interest here is that disclosed in Yokogawa Electric Works Bulletin 1F-2A1-G and referred to as the "YEWFLO" (Trademark) flowmeter . This flowmeter, which is described in more detail below, uses vortex tones to provide an indication of flow. A vortex shedder in the form of a trapezoidal rod extends across the pipe in which flow is to be measured takes place, with the large end of the vortex shedder facing upstream. A piezoelectric element is installed inside of the vortex shedder to detect vibratory stresses. The output signal produced by the piezoelectric element is processed and a resultant output proportional to the flow rate is produced and displayed. Other types of vortex flowmeters have also been used which employ different detection methods and different vortexing shapes.
A further arrangement of interest for determining fluid flow rate is that disclosed in U.S. Pat. No. 3,144,767 (Testerman et al). This patent discloses the use of an ultrasonic frequency wave generator as a fluid volumetric flowmeter. The Testerman et al patent provides for interrupting of a gas stream near the orifice, and for placing a cavity below, and preferably perpendicular to, the flowing jet for maximum sensitivity, so as to obtain an amplitude reinforcement of the undulations in the jet by varying the cavity length to a point where the system is caused to oscillate. The edge of the cavity opposite the orifice is said to serve as a means of perturbation of the flowing jet stream. In the embodiment illustrated in the patent, two adjacent blocks are provided with a threaded horizontal inlet passage, a communicating vertical passage and a horizontal orifice passage being formed in one block and a resonant cavity formed in the other block in communication with the orifice of the orifice passage so that the gas stream from the latter flows across the cavity. The patent describes frequency measurements for several gases and provides for measuring such frequencies by placing a microphone near the gas orifice.
Other patents of possible interest include U.S. Pat. Nos. 3,958,458 (Foreman et al); 3,930,556 (Kusuda et al) and 2,936,619 (Gibney). The Foreman et al patent discloses an acoustic emission flow measurement system wherein pseudo-sound created by fluid flow over an unspecified discontinuity is sensed by a transducer. The output of the transducer is reduced to a frequency-amplitude distribution and the amplitude at the characteristic frequency is used to calculate the flow rate. The Kusuda et al patent relates to leak detection, and noise corresponding to a leak is monitored by transducers, and a pair of signals cross-correlated, in attempting to determine the location of the leak. The Gibney patent relates to a device for measuring liquid flow wherein a serrated section is inserted into a pipe and a transducer is affixed to the pipe wall. The output voltage from the transducer is said to be related to the sound intensity and proportional to the flow rate. A constant frequency appears to be employed.